In general, the present invention is concerned with the reduction of noise in turbojet engines and in particular with the reduction of core and turbine noise by attenuating both of these noise components within the nozzle of a turbojet engine.
For a number of years there has been a concentrated effort in the aircraft industry to reduce the noise produced by commercial aircraft because of the disturbing effect it has on the inhabitants of communities located near those airports used by commercial carriers. Numerous studies of aircraft noise have identified the components that make up the total perceivable noise and steps have been made to eliminate such components or at least reduce intensity. For example, the aircraft industry has been successful in reducing jet noise, which is a noise produced by the mixing of high velocity exhaust gases with atmospheric air by switching to high-bypass turbofan engines. Such engines have lower fan and core exhaust velocities, and hence jet noise which is directly proportional to the exhaust velocities has been greatly reduced. Noise originating from the fan of turbofan engines has been partially suppressed by the use of acoustically absorbant liners strategically placed within the fan casing and fan exhaust duct. Similarly, turbine noise has been minimized by using acoustically absorbant liners located within the core exhaust nozzle of the turbofan engine.
However, reductions in jet, fan and turbine noise components have not yielded the overall reduction in noise that was expected by researchers. Further studies stimulated by this unexpected result have led to the discovery of still another noise component, namely, core noise, as reported by Bushell, K. "A Survey of Low Velocity Coaxial Jet Noise with Application to Jet Prediction", Symposium of Aerodynamic Noise, Sept. 1970. Although the exact origin of the core noise is still not well understood, it has been defined as the residual noise component that is left over after jet, fan and turbine noise have been identified and subtracted from the total rearwardly radiated noise. So far as the origin of core noise is understood, it is believed to include contributions from the combustion process within the combustor, interaction of the flow of gases from the combustor and the blades of the turbine and general flow noise as the exhaust gases flow past the structural parts of the engine's turbine and nozzle. The sound level verses frequency spectrum of core noise is essentially broadband, peaking at a relatively low frequency of around 200 to 800Hz where most of the energy of the core noise is concentrated.
Since most of the core noise energy occurs within a frequency range that is substantially lower than the range of frequencies of turbine noise, the low frequency end of which is typically around 2,000Hz, commonly used acoustically absorbant nozzle linings, tuned to the turbine frequency range, do not provide appreciable attenuation of core noise. Moreover, modification of the nozzle's acoustical lining in order to absorb the core noise is likely to diminish the effectiveness of the lining for attenuating the higher frequency turbine noise components. Thus, one of the problems in reducing core noise, is the fact that any new or modified acoustically absorbant structure for the nozzle must be compatible with noise absorbant structures tuned to the higher frequency turbine noise. A related problem is the design criteria imposed by limited space available within the engine's nozzle for placement of both core and turbine sound absorbing structures. While in other environments it may be possible to use two physically separate, serially arranged sound absorbing structures, one tuned to the core noise frequencies and the other tuned to the turbine noise frequencies, the space limitation within the nozzle does not permit such cascading of structures.
Furthermore, the high pressures, high temperatures and severe temperature gradients that exist within the core exhaust nozzle impose additional constraints on the types of acoustically absorbant structures that can be used. For example, it is not practical to use sound absorbing structures that employ fiberous material because of the susceptibility of such material to disintegration in the presence of the high gas temperatures and high energy vibration, i.e., high level sound. Also, large temperature gradients, existing within the nozzle, especially during engine start-up cause differential thermal expansion of the metal parts and the acoustically absorbant structures must be able to accomodate such temperature effects without developing dangerous localized stresses within the nozzle.
Other nozzle design criteria such as duct geometry, the amount of drag caused by the surface characteristics of the duct wall, overall nozzle size and weight, nozzle discharge capability and ease of fabrication impose further constraints on the type of the noise absorbing structure that can be employed.
Accordingly, one object of the present invention is to provide a nozzle assembly for the core of a turbojet engine, wherein such assembly includes sound absorbing structure that is effective in attenuating both core noise and turbine noise.
A further object of the present invention is to provide a turbojet core nozzle assembly that has an acoustically absorbant structure which is compatible with certain minimum stress design requirements typically specified for nozzles of this general type.
Still another object of the present invention is to provide a turbojet core nozzle assembly that has the above-mentioned acoustically absorbant characteristics and which is compatible with other nozzle design criteria including duct geometry, overall size, nozzle weight, nozzle discharge and thrust, and ease of fabrication.